Lecture-13 - Specificity of drug action No drug is entirely...

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Specificity of drug action No drug is entirely specific in the sense that it acts exclusively only on one type of cell or tissue, having just the desired effect and no other. Drugs vary in their specificities and the usefulness of a drug clinically is often directly related to its specificity. Poison : a compound which has deleterious effects on cell function without having any therapeutic effects. Eg., cyanide combines strongly with the Fe 3+ found in many proteins interfering in their functioning. Some drugs have absolutely no toxicity at concentrations used clinically. Eg., penicillin inhibits a bacterial enzyme involved in the formation of bacterial cell walls. Humans, lacking cell walls, are unaffected by these concentrations of penicillin.
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In between these two extremes (cyanide & penicillin) are many drugs used clinically. Methotrexate is a drug used in cancer chemotherapy and to treat severe cases of psoriasis (using doses of 2.5 - 5 mg/kg). It acts by inhibiting the rapid reproduction of epithelial cells in psoriatic plaques. However, at slightly higher doses, methotrexate also inhibits reproduction of mucosal cells in the intestine, which would lead to ulceration and diarrhea. Thus, useful drug actions are instances of selective toxicity while non- selective toxicity gives rise to poisoning .
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Generally, the useful, therapeutic effects of drugs are separable from the toxic effects based on differences in their respective mechanisms of action their dose-response relationships if their mechanisms of action are similar. the sites at which therapeutic and toxic effects are produced. Attempts to increase the utility of a drug are based on improved pharmacodynamic specificity (if the mechanisms of toxic and therapeutic effects differ) or an enhanced pharmacokinetic selectivity (distribution to the desired target site).
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For specific drug:receptor interactions to occur, the drug molecule must have several points of attachment to corresponding points on the receptor molecule. The nature of these points of attachment and their relative positions and distances apart are all critical for the drug’s ability to combine with a receptor and to produce a response. Molecular features necessary for acetylcholine action: 1. Positively charged N 2. Three CH 3 groups attached to N 3. Ester linkage 4. Spacing between N and carbonyl C
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Acetylcholine has actions at muscarinic and nicotinic acetylcholine receptors, but many other drugs act at one but not the other. Why?
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Lecture-13 - Specificity of drug action No drug is entirely...

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